1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor for use in laser printers, digital copiers and laser facsimiles. The invention also relates to a corresponding electrophotographic image forming apparatus and method, and a corresponding image process cartridge.
2. Discussion of the Background
Electrophotographic image forming devices can produce high-quality images at a high-speed, and are used for copiers and laser beam printers. An organic photoreceptor using an organic photoconductive material has been developed and has gradually become widely used as a photoreceptor in electrophotographic image forming devices. Over time, the photoreceptor has changed from a) a charge transporting complex constitution or a single-layered constitution wherein a charge generation material is dispersed in a binder resin to b) a functionally-separated constitution wherein a photosensitive layer is separated into charge generation layer and a charge transport layer, and has improved its performance. The currently prevailing approach includes use of a functionally-separated photoreceptor having a constitution wherein an undercoat layer is formed on an aluminum substrate, a charge generation layer is formed on the undercoat layer and a charge transport layer is formed on the charge generation layer.
In conventional systems, the undercoat layer is formed to improve adhesiveness, coatability, chargeability of the photosensitive layer, and to prevent an unnecessary charge from the substrate from entering the photosensitive layer and cover a defect on the substrate. The undercoat layer typically includes only a binder resin and an undercoat layer including a binder resin and a pigment.
Specific examples of resins used in the undercoat layer include water-soluble resins such as polyvinylalcohol and casein; alcohol-soluble resins such as nylon copolymers; and hardened resins having a three-dimensional network such as polyurethane, melamine resins, phenol resins, phenol resins, oil-free alkyd resins, epoxy resins and siloxane resins.
Although water-soluble resins are inexpensive and have good properties, a solvent for a photosensitive layer coating liquid dissolves the water-soluble resins and frequently deteriorates a coatability of the undercoat layer. Nylon alcohol-soluble resins are highly sensitive to environment because of their high water absorbability and affinity, and therefore the resultant photoreceptor changes its properties according to humidity.
In an atmosphere of high humidity, a photoreceptor having an undercoat layer using alcohol-soluble resins, particularly the nylon resins, absorb a large amount of water in the undercoat layer, and therefore properties thereof change significantly when repeatedly used in an environment of high temperature and high humidity or a low temperature and low humidity. This results in production of abnormal images such as black spots and deterioration of image density.
It is well known that an inorganic pigment such as titanium oxide may be dispersed in the undercoat layer to enhance a hiding effect of the defect on the substrate and a scattering effect of incident light such as coherence light (a laser beam) to prevent occurrence of an interference pattern. However, the above-mentioned deficiency in the face of humidity does not change even when the inorganic pigment is mixed with the nylon resins.
Among hardened resins having a three-dimensional network, a large amount of formaldehyde is used to form melamine resins, alkyd/melamine resins, acryl/melamine resins, phenol resins and methoxymethylated nylon. Therefore, unreacted materials are absorbed in the resins and the formaldehyde generates in a heat cross-linking process after the undercoat layer is formed. However, formaldehyde is an indoor pollutant listed in the Clean Air Act and is said to be a cause of an illness known as “sick house syndrome.” Thus, to prevent formaldehyde from being discharged to the atmosphere, expensive collection equipment needs to be used.
Therefore, there exists a demand for a less environmentally-damaging heat-crosslinking resin for use an undercoat layer, where the resin does not generate formaldehyde when hardened with heat. Specific examples of the resins include urethane resins. To harden the urethane resins, a compound, including a group including an active hydrogen such as acrylpolyol, is dried with hot air for a predetermined period of time in the presence of a hardener, such as a monomer including an isocyanate group, such that a three-dimensional network crosslinking reaction between the group including an active hydrogen of the acrylpolyol and isocyanate group of the hardener starts to form a hardened film. However, since the isocyanate group has a high reactivity, a coating liquid using the isocyanate group has a short usable time. Therefore, a block isocyanate having a long pot life in a coating liquid for an electrophotographic photoreceptor and an isocyanate coating material, which is stable in the presence of alcohol-soluble chemicals, water-soluble chemicals or the compound including a group including an active hydrogen, is a topic of ongoing research.
The block isocyanate includes an isocyanate group protected with a blocker such as oxime and starts an addition reaction with a compound, including a group including active hydrogen such as a hydroxyl group, when heated and the blocker is removed to proceed a crosslinking reaction.
However, coherent light makes a regular reflection on a surface of the undercoat layer using the oil-free alkyd resin and block isocyanate, resulting in occurrence of moire. Therefore, the coherent light is forced to scatter thereon.
To prevent the moire and interference pattern, Japanese Laid-Open Patent Publications Nos. 11-65156 and 2000-171997 disclose an electrophotographic photoreceptor including a surface-treated or a resin-coated particulate material in its undercoat layer. However, when the particulate material is coated with a resin, a residual potential of the resultant photoreceptor increases, which is considered to be caused by increase of a surface resistance of a pigment in an environment of high temperature and high humidity. Thus, when the resultant photoreceptor is installed in an image forming apparatus, black part potential increases, resulting in deterioration of image density.
Japanese Laid-Open Patent Publication No. 11-202518 discloses an electrophotographic photoreceptor including a zirconium oxide in an amount of not less than 20% by weight in its undercoat layer. However, the electrophotographic photoreceptor probably has similar problems to the above problems.
Japanese Laid-Open Patent Publication No. 2000-181113 discloses an organic photoreceptor drum including a layer in which an electroconductive powder is dispersed. However, when a photoreceptor includes at least the electroconductive powder-dispersed layer and a photosensitive layer, a low-resistance pigment used as the electroconductive powder and a charge generation material are probably present while contacting with each other. Therefore, a charge easily enters the photosensitive layer and a surface charge leaks at the contact point when the photoreceptor is charged. In addition, when the organic photoreceptor drum is installed in a reverse development image forming apparatus, images having black spots in their blanks tend to be produced.
Japanese Laid-Open Patent Publication No. 6-236061 discloses an electrophotographic photoreceptor including a white pigment and an airspace of not less than 10% by volume fraction in its undercoat layer. However, when a photosensitive layer is coated and dried thereon, the airspace causes an air bubble in the photosensitive layer, resulting in a defectively coated photosensitive layer. In addition, JP 6-236061 discloses that a resin layer is formed between the electroconductive-powder-dispersed layer and photosensitive layer. However, the resultant photoreceptor properties largely change depending on the resin, and the resin layer therebetween results in a new photosensitive layer which causes a higher cost.
Therefore, as discovered by the present inventors, a demand exists for an improved electrophotographic photoreceptor apparatus and method which does not produce images having the moire or interference pattern.